This invention relates to a slip-controlled brake system for automotive vehicles with a driven axle and a non-driven axle. The brake system has a master cylinder at whose working chamber there are connected a pressure medium storage/pressure compensation reservoir, an auxiliary pressure supply system which can be switched on in the control phases, and wheel brakes. In the connection of the working chamber to the reservoir a control valve is provided which in the brake released position of the master cylinder piston is open towards the reservoir, and which closes upon the actuation of the brake with the auxiliary pressure supply switched off. During metering-in of pressure medium from the auxiliary pressure supply system the control valve maintains a pressure in the working chamber of the master cylinder, which pressure is proportional to the force of the brake pedal.
The hydraulic brake system of this type is described in German published Patent Application No. P 36 01 914. That system has a master cylinder with a vacuum brake booster connected upstream and is equipped with an auxiliary pressure supply system. Connected to each of the working chambers within the master cylinder are one or several wheel brakes, the auxiliary pressure supply system and by way of a control valve, a pressure compensation reservoir. The control valve is open and establishes a communication towards the reservoir in the released position of the piston within the master cylinder when the brake is not operated. Upon brake application, the valve closes and remains closed as long as the pedal-actuation-released pressure in the working chamber of the master cylinder is lower than the auxiliary pressure. Upon the onset of slip control, the auxiliary pressure supply system is switched on and pressure medium is metered into the working chamber. Thereby the piston in the master cylinder is returned to its at rest position. The control valve is opened and remains opened until the forces on the piston have balanced. Even after the switching-on of the auxiliary pressure supply system, a controlled pressure prevails in the working chamber. The pressure is proportional to the force of the pedal. Upon an instabilization of any wheel, the braking pressure is varied by means of change-over multi-directional valves, namely inlet valves and outlet valves, inserted into the pressure medium paths from the wheel brakes to the master cylinder and to the reservoir, thus controlling the brake slip. This sytem, however, is not suited for traction slip control commonly referred to as "TSC".
There is already known a brake system with a hydraulic brake booster where, for traction slip control, the wheel brakes are directly connectible to the auxiliary pressure supply system by way of multi-directional valves. Such a system is disclosed in the published German published patent applicatio DE-OS No. 34 07 538. In that system, the braking pressure generator is a dual-circuit-type in which each circuit is connected to one driven and one non-driven wheel. During traction slip control the pressure medium path to the non-driven wheel is blocked by an additional multi-directional valve. Systems of this type are relatively intricate and expensive.
Another system provides a brake system having a tandem master cylinder with a vacuum brake booster connected upstream and an auxiliary pressure supply system which is switched on only during the control phases and which, instead of the braking pressure generator, can be connected to the wheel brakes by means of multi-directional valves. Such a system is disclosed in German Published Patent Application No. P 35 27 190. This brake system also requires a relatively intricate and expensive valve arrangement.